"The Weakness"
Well, the only problem I have with that question is how it's phrased, and that's basically not a problem because anyone with a working brain will know what you mean by asking this. It's not like every champion has one, hard-set, hard-counter weakness; rather, they tend to have a collection of strengths that come with their own weaknesses.
Yasuo has a couple of solid defense tools, but one of them goes away the second that combat starts, and the other doesn't block near as many things as people would have you believe. Both of them take a significant amount of time to re-prep once they've been used, since people generally max W last and his shield passive requires an average 10 seconds of running/dashing around to refresh. Even in the worst late-game scenario, you still have at least a 5-second window for getting past those defenses, which is a good bit of time for retaliation.
Early game, just make sure to poke that shield if you have any way to do it without going into a full engagement.
Early on, his DPS isn't really that spectacular; it relies on him landing his Qs (it's surprising how well kiting works against a melee) and most players tend to rely on him getting in bursts of E-Q damage and running away from retaliation. Position yourself so he'd have to overextend to reach you. Standing to the side of the minion wave and pulling him away severely cuts down his mobility.
inb4 "CC is everyone's weakness," Yasuo's on the list of supremely susceptible to CC champions. The kind of champion where a stun, or even a lengthy slow, can spell death in seconds. If you have it, save your CC, time it right, it will make a huge difference. Or if you happen to have really spammy CC, use that to zone him harshly.
Speaking of which, Stars Shaper mentioned this as well, but Yasuo relies on being able to control the lane. If he can gain control early game, he can get his lead and become a serious wrecking ball like any carry. Hopefully, your choice of laning opponent is capable of zoning as well, because you're going to need to out-control the lane.
And, seriously, dodge those Qs. Sure, you can't dodge 100% of skill shots, but every dodge makes a difference.
The most important thing to remember when trying to learn a champion's weaknesses is to play the champion for yourself. Advice can help, but only so much. When you actually play as a champion, you learn how their kit works, and you start learning how people fight against them. It might take more time, but it definitely solidifies your understanding of champions.